Field of the Invention
The present disclosure relates to a display device and a light unit used for the same. More particularly, the present disclosure relates to an ultrathin light unit for a display device that is capable of providing a collimated light.
Discussion of the Related Art
Recently, a variety of technologies and researches for making and reproducing three-dimensional (3D) image/video have been actively carried out. Media relating to a 3D image/video is a new concept for virtual reality that is capable of further improving visual information and expected to lead to next generation display devices. A conventional two-dimensional (2D) image system merely reproduces an image and video data into a plan view, but a 3D image system can provide a full real image data to an observer. For this reason, 3D image/video technologies are the True North image/video technologies.
Typically, there are three methods for reproducing 3D image/video: a stereoscopy method, a holography method and an integral imaging method. Among these methods, the holography method uses laser beams so that it is possible to observe 3D image/video with naked eyes. The holography method is the most ideal method because it has an excellent visual stereoscopic property without any fatigue of the observer.
To produce the recording of a phase of a light wave at each point in an image, holography uses a reference beam which is combined with light from the scene or an object (an object beam). If these two beams are coherent, an optical interference between the reference beam and the object beam, due to the superposition of the light waves, produces a series of intensity fringes that can be recorded on a standard photographic film. These fringes form a type of diffraction grating on the film, which is called hologram. The central goal of holography is that when the recorded grating is later illuminated by a substitute reference beam, the original object beam is reconstructed (or reproduced), realizing a 3D image/video.
When a display system is implemented using a holographic technology according to the related art, it may be difficult to obtain evenly distributed brightness because an intensity of light radiated from a light source follows the Gaussian Profile. In addition, when the incident light from the light source has an inclined incident angle in order to reduce the high order diffraction components causing an image noise, a collimation degree of the laser may be reduce.
In order to address these drawbacks of the related art, researches have been made to provide a light unit that can provide a collimated light even when the incident light has an inclined angle for reducing high order diffraction components. For example, a system using a collimation lens has been presented.
FIG. 1A schematically illustrates a structure of a light unit that can provide a collimated light using a collimation lens according to the related art.
Referring to FIG. 1A, by disposing a point light source 30 at the position of the light source and positioning a collimation lens CL at the focal length position apart from the light source 30, the light radiated from the point light source 30 can be formed as a collimated light beam. This collimated light beam can be used as a reference light beam in a non-glasses type display system.
In most holographic display systems, it is, however, preferred that the reference light beam is incident on the diffraction optical element with an inclined angle from a vertical direction to the incident surface of the diffraction optical element. This is because, as the diffraction element such as a holographic film may generate the 0th mode image and/or 1st mode image that may work as noises in the holographic image, the 0th mode and/or the 1st mode can be reduced or eliminated by making the reference light beam being incident onto the diffraction element with an inclined angle. For example, the position of the point light source 30 may be shifted at any one side to make an inclined angle in the light unit shown in FIG. 1A.
FIG. 1B schematically illustrates a structure of a light unit generating a collimated light beam using a collimation lens, in which the collimated light beam has an inclined angle, according to the related art.
Referring to FIG. 1B, the point light source 30 is shifted or moved to upside from the light axis 130 so that the inclined angle from the light axis forwarding to the center of the lens CL may be α. Theoretically, as indicated by the dotted lines in FIG. 1B, the collimated light beam has the inclined angle α from the light axis 130. However, in actual cases, due to physical characteristics such as a spherical aberration, the real light path may not be collimated and/or paralleled with the inclined angle α, as indicated by the solid lines in FIG. 1B. As a result, the light beam from the light unit BLU may not be incident into the desired area and/or direction evenly, but be unevenly distributed over the incident surface of the diffraction optical element.
To address this problem, a method of combining the collimation lens with a prism sheet has been proposed to control the direction of the light from the light unit. Such a light direction controllable light unit is briefly described below with reference to FIG. 2.
FIG. 2 schematically illustrates a structure of a light unit that provides a collimated light beam of which direction can be controllable according to the related art.
The light direction controllable light unit BLU according to the related art comprises a collimated lens CL, a point light source 30 disposed at one side of the collimation lens CL and a prism sheet PS disposed at the other side of the collimated lens CL. The point light source 30 may be any type of light source that can radiate light in radial directions from one point. In order to direct most of the light from the point light source 30 to the collimation lens CL, a minor (not shown) may be further included at the back side of the point light source 30.
The point light source 30 can be preferably disposed at the focal plane of the collimation lens CL. Especially, the point light source 30 can be positioned on the light axis 130 connecting between the center point of the collimation lens CL and the center point of the focal plane of the collimation lens CL.
The collimation lens CL may change the light radiated from the point light source 30 into a collimated light beam 100. That is, the collimated light beam 100 may radiate in one direction parallel to the light axis 130. The collimation lens CL may include any optical lenses such as a Fresnel lens.
The prism sheet PS is preferably positioned opposite the point light source 30 with the collimation lens CL interposed therebetween. The prism sheet PS may refract or change the light propagation direction with certain angle α as being inclined with respect to the light axis 130. With the prism sheet PS, the parallel property of the collimated light beam 100 is maintained, and the propagation direction of the collimated light is redirected downward with an angle of α with respect to the light axis 130. As a result, the prism sheet PS can change the collimated light beam 100 into the controlled collimated light beam 200. The prism sheet PS may include a Fresnel prism sheet.
The light unit described above can be applied to a hologram 3D display or an ultrathin flat panel display such as a controlled viewing window display and so on. Particularly, the ultrathin flat panel display can be applied to various display systems. For example, as the viewing window can be controlled, it can be applied to a security display system in which display information is presented only to specific persons. As for another example, it can be applied to a multi-viewing display system in which different video data can be provided to different positions (or ‘viewing areas’). Further, as the left eye image and the right eye image can be respectively provided to the left eye and the right eye without any interference, a good 3D display can be designed.
FIG. 3 schematically illustrates a structure of an ultrathin flat panel display according to the related art.
Referring to FIG. 3, the ultrathin flat panel display according to the related art comprises a display panel LCP representing video data and a light unit BLU. The display panel LCP may be a flat panel display using a light system, such as a liquid crystal display panel. The ultrathin flat panel display directs the display information represented on the display panel LCP into a certain area or specific viewing window. In order to control the viewing window, the light unit BLU is desired to control the radiating area of the light. For example, the light unit BLU may adopt the light control system shown in FIG. 2.
In detail, the light unit BLU for the ultrathin flat panel display according to the related art may include a light source LED, a lens LEN, a reflection plate REF and a holographic film HOE. To implement a holographic technology, it is preferable to use a highly collimated light beam, with the light source LED being a laser or a light emitting diode laser. When the light source LED is a general light emitting diode, a collimation lens LEN may be further included to obtain a collimated light beam. The holographic film HOE is to make the collimated light radiating to a specific viewing area. By radiating the light as a reference light beam onto the holographic film HOE, the light of which radiating area can be controlled according to the recording pattern of the holographic film HOE can be provided to the display panel LCP.
In order to develop a large-area ultrathin flat panel display, a large-area holographic film HOE is desired to be disposed at the back side of the large-area display panel LDP. Further, a reflection plate REF may be included to direct the light radiated from the light source LED and collimated by the collimation lens LEN to the large-area holographic film HOE.
As mentioned above, the ultrathin flat panel display includes the lens LEN and the reflection plate REF for optically converging and diverging the light. As a result, a physical space for ensuring enough light path may be required in order to provide a highly collimated light. That is, the light unit BLU according to the related art may require a large volume space, and thus, the ultrathin flat panel display according to the related art may also have a large volume space, thereby making it difficult to apply them to various display systems. Further, the light unit BLU may have a limited field of a controlled viewing window, thereby making it difficult to use as a general-purpose light unit.